As if. A rough pragmatic equivalent perhaps, in formal English, but their syntax is quite different: as if always heads a tensed clause, while as it wereis a clause, and a parenthetical one at that. Only in very informal English does one find parenthetical as if functioning this way.
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John LawlerJan 31 '12 at 18:16

@JohnLawler, sorry, I take it that you disagree with something I say above, but I can't quite tell what... could you clarify?
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senderleJan 31 '12 at 19:02

Your last sentence: "A rough grammatical equivalent in modern English might be 'as if,' as in 'he reeled as if hit by a sledgehammer.'" Pragmatically equivalent and semantically equivalent in some ways, but 'grammatically equivalent' is precisely what it's not. That's all.
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John LawlerJan 31 '12 at 21:01

@JohnLawler, I wasn't trying to say "as if" in modern usage is equivalent to "as it were" in modern usage, as exemplified by the quotations from c1400 on. I was trying to say that "as if" in modern usage is roughly equivalent to "as it were" as used in the first quote from 1135, which I've seen rendered "the sun became such as if it were a three-nights’ old moon." But perhaps that rendering is wrong, or perhaps I'm misinterpreting things...
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senderleJan 31 '12 at 21:54

1

@JohnLawler, well, I completely understand you there. I guess I'll change it just to be safe.
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senderleJan 31 '12 at 22:05

In general, we encourage answerers to support their arguments with references to external authorities or evidence. In other words, "to my mind" is not generally speaking enough to warrant posting an answer.
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Dan BronMay 4 at 14:13

This does not explain the origin of the phrase, which is what the question asks for. They already know the meaning in terms of other phrases.
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Matt E. Эллен♦May 5 at 15:53